1. Field of the Invention
This invention relates to an electromagnetic energy seal for preventing leakage of electromagnetic waves through a gap between a door and a heating chamber formed in the body of a microwave oven or the like, and more particularly to a choke cavity arrangement having an electromagnetic energy sealing function both for the fundamental frequency electromagnetic wave used for heating and the higher harmonic electromagnetic waves.
2. Description of the Related Art
A magnetron is now most commonly used as an oscillator tube in an apparatus such as a microwave oven, because it operates at a high oscillation efficiency and has a simple structure. However, the magnetron generates, in addition to the microwave frequency of 2,450 MHz which is the fundamental frequency of the microwave energy used for heating, higher harmonic microwaves having frequencies n times (n=positive integers) as high as the fundamental frequency.
Various communication techniques have recently been developed, and the utilization of microwave frequency bands in the communication field has been developed. Especially, in the past several years, the use of a frequency range in the vicinity of 12.25 GHz, which is the fifth higher harmonic of 2,450 MHz, in the DBS (a direct broadcasting system) has been promoted. Because of such a trend, the suppression of the leakage of higher harmonic microwave components from a microwave oven or the like has been demanded to an extent greater than before, and, especially, the possibility of impartation of actual harm to the DBS by the fifth harmonic microwave component is now drawing the attention of those who have a concern in this communication field.
A door of a microwave oven is provided with a choke cavity at an outer peripheral part of the door which is opposite to the peripheral edge of the entrance opening of the heating chamber when the door is closed, so that an electromagnetic wave may not leak to the outside through a gap between the heating chamber and the door in the closed position of the door. However, although this choke cavity exhibits a great choking effect mainly against the fundamental frequency wave of 2,450 MHz, it exhibits almost no choking effect against the aforementioned higher harmonic microwave components, especially, the fourth or fifth higher harmonic component. Therefore, the provision of a second and a third choke cavity has been proposed for the purpose of preventing the leakage of such higher harmonic microwave components. (The arrangement of such choke cavities are disclosed in, for example, Japanese Examined Utility Model Publication No. 48-4121, Japanese Examined Utility Model Publication No. 48-5070, Japanese Examined Utility Model Publication No. 52-7880 and Japanese Examined Patent Publication No. 52-3126.)
However, a microwave oven provided with such prior art choke cavities, in which individual choke cavities are disposed independently of one another, becomes complicated in structure and large in size, and has not been satisfactorily suitable for practical use from the view points of manufacturing cost and design.